Foams based on polyisocyanates, for example, polyurethane, polyisocyanurate and polycarbodiimide foams, are widely known and used. The main methods of producing these foams are the continuous block foaming method and foaming in molds.
There have been many attempts to improve the known properties of these foams. Thus, for example, the incorporation of solid and liquid flame retarding agents during the foaming reaction has been described in the literature for improving the flame resistance of polyurethane foams. These measures, however, can only provide a slight improvement in the flame resistance properties. Since these flame retarding agents are foreign materials which do not, in most cases, take part in the reaction, the resulting foams are often inferior in their mechanical properties compared to foams which have not been flame-proofed. Also, these flame retarding agents are frequently insoluble and have the effect of increasing viscosity. Liquid flame retarding agents affect the foaming reaction due to their basic or acid character. Because of this, they cannot be used in all desired proportions and/or incorporated in every formulation. Solid flame retarding agents can only be foamed with considerable technical difficulty because they must first be made up into a paste and therefore are impossible to deliver from some types of feed apparatus.
For the above reasons and due to their deleterious effect on the mechanical properties of the foams finally obtained, the quantities in which these agents are used in proportion to the other reactants is limited. Furthermore, by far the majority of known flame retarding agents are halogen compounds or halogen-phosphorus compounds. These types of compounds may develop toxic and/or corrosive gases in the event of fire. This does not eliminate the dangers; it only displaces them.
Attempts have also been made to improve the mechanical and flame retarding properties of foams by impregnation. Thus, a process for the impregnation of lightweight foams with solutions of hardenable polyurethanes is described in U.S. Pat. No. 2,955,056. Although the process makes it possible for foams with improved mechanical properties to be produced, it has the serious disadvantage of requiring the use of organic solvents. In many cases, in practice, these solvents are difficult to handle because they are often combustible and/or highly flammable and produce explosive gas mixtures with air. They are many times physiologically harmful also. Furthermore, foams which have been impregnated in this way have the disadvantage that, due to the organic solvent used, they undergo an undesirable initial swelling and after evaporation of the solvent, the foam is often left behind as a shrunken mass with occluded cell structure.
The flame proofing of foams by impregnation with polychloroprene latex containing filler has been described in British Pat. No. 977,929. Disadvantages of this process are the high halogen content of the flame retarding mixture and consequent high density of smoke, the cross-linking with sulphur required to fix the impregnating substance and the fact that the flame-proofing obtained is in many cases insufficient.
According to German Pat. No. 2,031,160, the mechanical properties, lightfastness and flame resistance of open celled foams which have a low unit weight can be improved by impregnating the foams with aqueous dispersions of rigid elastic or flexible polyurethane resins. This patent also describes the use of fillers in general. It has now been found that aluminum hydroxide is a particularly valuable filler, but its use is not described in the above-mentioned German patent.